This invention relates to heat exchangers, and more particularly to recuperators in which hot gases such as exhaust gases of combustion or flue gases are used to preheat air or fluid fuel to be used in combustion.
In recent years the greatly increased costs of fuel, labor and capital equipment have made it important to recover as much heat as possible from flue gases and transfer it to the incoming combustion air in order to increase the heating efficiency. It is also important that furnace shutdowns for maintenance, such as for repair or replacement of recuperators, be reduced to a minimum. While new furnace installations are designed and built with these factors in mind, great efforts are also being made to increase the efficiency and outputs of existing furnaces because of high costs of new furnaces and long times required to build and put them into operation; and increased recovery of otherwise wasted heat in flue gases is one of the most effective and economical approaches toward achieving increases in efficiency and output of old furnaces.
For these reasons it is desired that recuperators employed not only provide high recovery of heat from flue gases, but also have long service lives with a minimum of maintenance and furnace shut-down. This, however, is exceedingly difficult to achieve because of the extremely rigorous service conditions to which the recuperator heat exchange elements are subjected in operation.
In recuperators of the general type in connection with which this invention provides particular advantages, i.e. metallic recuperators having hollow plate type heat exchange elements, the heat exchange elements are exposed to hot waste or flue gases which often are at such high temperatures that they are incandescent, and which usually have high velocities. The high temperatures alone have deteriorating effects on the metal of the outer walls of the heat exchange elements; and the relatively high velocities of the hot gases also tend to harmfully affect the metal by erosion tendencies. The flue gases also contain constituents which tend to corrode or cause other deleterious chemical or metallurgical reactions on such metal, and this action is promoted by the high gas temperatures and velocities.
In the heat exchange elements these harmful effects occur to the greatest extent on the portions of the outer walls of the heat exchange elements facing the flow of hot flue gases.